This invention relates generally to a panel used for attenuating sound frequencies and more particularly but not by way of limitation to a cellular honeycomb core sandwich construction for installation in an engine nacelle or other applications requiring sound absorption.
Heretofore advanced technology sound attenuating panels in an engine nacelle have been constructed primarily using a double layer of honeycomb core with a septum bonded between the two layers. Facing material is then bonded on both sides of the double layer forming a core sandwich. This type of construction requires four bond lines which degrade from the strength of the sandwich construction. Further a double layer core construction was used for attenuating both high frequency sound and low frequency "buzzsaw" noise which required a short section of lining with increased depth causing significant fabrication complications. This type of construction required added weight due to both the additional depth of the core and special attachment flanges necessary to secure the two linings together.
Also, the surface area required by the additional "buzzsaw" lining is not effective for high frequency noise suppression. This resulted in a compromise between noise suppression of the low frequency "buzzsaw" noise and the high frequency broadband and tone noise.
The subject invention is an improved design utilizing a continuous buried septum which is described in U.S. Pat. No. 4,265,955 and U.S. Pat. No. 4,257,998. These patents disclose a honeycomb core with internal septum and a method of making a cellular core with internal septum. The patents are assigned to the assignee of interest of this application and the patent references cited during the prosecution of the above mentioned patents are incorporated herein by reference.